I have a 1999 Volvo S70, the speedometer works on and off mostly off and the brake, check engine aqnd ABS lights remain on.

Output Speed Sensor No Signal
http://www.autocodes.com/p0722_volvo.html

yes there is a code table here: code table

This is probably a fault with the ABS control unit. Is your ABS light on?

i would check the speed sensor

The bad news is the only part that is connected to all of these is the ECM or computer.

Most likely an ABS control unit, have the system scanned first.

Change the speed sensor in the top of the rear differential

Usually the ABS computer knows what it is doing. You may want someone to hookup a scanner to the ABS test port.
When I had my speedometer and odometer act up, it turned out to be a bad alternator. You might want to check yours.

Sounds like computer to me , black smoke might just be due to fuel /air mixture.
Avoid dealerships , find a reputable mechanic.
Almost any shop can plug your car into a diagnostic computer .
If you live close to me , I would be happy to take an s-70 off your hands :)

There are two problems here. First you need and abs control unit. They are very expensive and it is possible to rebuild them. A couple of places to call: ATE, Auto Truck Electronics, they are in Florida, and BBA Remanufacturing near Boston, MA. I have used both companies and they charge much less to rebuild them than it costs to purchase a new one. That should take care of the speedometer, brake light, and ABS light. As for the check engine, that could be anything and the car will need to be scanned. ETS light can be remedied by a trip to the dealer. There is a campaign to install new software to the Electronic Throttle Controller. This is no charge to the customer and I believe the car has to be under 200,000 miles.